A) Field of Invention
The present invention relates to a bearing device for use in a motor, and more particularly to a dynamic pressure bearing device for use in a motor which prevents vibrational movement of a shaft in the motor and minimizes a loss of torque in the motor.
B) Description of Related Art
FIG. 1 shows a conventional dynamic pressure bearing device for use in a motor for a hard disk drive. Referring to FIG. 1, a fixed member or frame 1 is shown to include an integral bearing holder 1a which extends in an upright direction and is cylindrical in shape. The frame 1 with the bearing holder 1a has a concave shape, with a closed end at a bottom of the frame 1 and an open end at a top of the frame 1. A stator core 6 is fixed to an outer peripheral surface of the bearing holder 1a and a coil 5 is wound on the stator core 6.
Two radial-slide bearings 2 are sectored to an inner periphery of the bearing holder 1a where a center shaft 3, which revolves during operation of the motor, extends through the radial-slide bearings 2. A dynamic-pressure groove is formed in either an outer peripheral surface of the center shaft 3 or an inner peripheral surface of each of the radial-slide bearings 2. The dynamic-pressure groove has, by way of example, a herringbone shape. An oil or noncompressible fluid is filled in a space between the center shaft 3 and the radial-slide bearings 2. Under this arrangement, a radial-dynamic pressure is developed between the center shaft 3 and the radial-slide bearings 2 so as to prevent radial vibrational movement of the center shaft 3 when it is rotated within the radial-slide bearings 2.
An annular thrust plate 13 is fit around a lower end of the center shaft 3 which is adjacent to the closed end of frame 1. The annular thrust plate 13 has an outer diameter which is greater than an inner diameter of the radial-slide bearings 2, as measured from the a center of the center of shaft 3. The annular thrust plate 13 has an end surface 13a which is substantially flush with an end surface 3a of the center shaft 3. A dynamic-pressure groove is formed in either (i) the lower end 13a of the annular thrust plate 13 and the Connected lower end 3a of the center shaft 3, or (ii) a bottom 1b of the frame 1. A dynamic pressure groove is also formed in either an upper end 13b of the annular thrust plate 13 or a lower end 2a of the radial-slide bearing 2 located adjacent to the closed end of the frame 1.
A lubricant is filled in both a clearance formed between the upper end 13b of the annular thrust plate 13 and the radial-slide bearing 2 and a clearance formed between the lower end 13a of the annular thrust plate 13 and the bottom 1b. That is, the lubricant is filled in a space 15 within which the annular thrust plate 13 is received. As a result, a thrust-dynamic pressure is developed between (i) the lower end 13a of the annular thrust plate 13 and the connected lower end 3a of the center shaft 3, and (ii) the bottom 1b of the frame 1. A thrust-dynamic pressure is also developed between the upper end 13b of the annular thrust plate 13 and the lower end 2a of the radial-slide bearing 2 adjacent to the closed end of the frame 1. These pressures prevent vibration of the center shaft 3 in a thrust direction. Thus, the center shaft 3 is well balanced when it is rotated on the bottom 1b of the recess.
A hub 4 is fit on an upper end of the center shaft 3 adjacent to the open end of the frame 1 and is shaped as to cover the stator core 6 and the coil 5. Although not shown, disks are loaded on an outer periphery of the hub 4. A drive magnet 7 is fixed to an inner periphery of the hub 4 and is positioned to face the stator core 6.
A passage 10 is defined between an interior and exterior of the bearing holder 1a. A magnetic-fluid seal 8 is present in the passage 10 above the radial-slide bearing 2 near the open end of the frame 1. The magnetic-fluid seal 8 includes a magnet 8b, and a pair of pole pieces 8a. The pole pieces 8a are adapted to sandwich axial ends of the magnet 8b and form a magnetic passage. A magnetic fluid 9 is held between an inner peripheral surface of the pole pieces 8a and an outer peripheral surface of the center shaft 3. The magnetic-fluid seal 8 is present to prevent leakage of the lubricant placed within the bearing holder 1a, and to prevent the entry of dust or other particles into the interior of the bearing.
A predetermined drive voltage is applied from an external power source, not shown, to the coil 5 through a flexible board 12 so as to rotate the hub 4 with the loaded disks. Also provided is a bearing collar 11 which is sandwiched between the radial-slide bearings 2.
In order to resist vibration and shock and thus effectively read and write data on the disks, the motor is designed to maintain the thrust-dynamic pressures as described above. For example, if a shock or vibration is experienced which pushes the center shaft 3 in the downward direction, the volume of the space 15 beneath lower ends 3a and 13a is reduced by an amount proportional to a cross sectional area of the center shaft 3 and annular thrust plate 13. This loss in volume causes the lubricant within the space 15 to flow toward the radial-slide bearing 2 near the closed end of the frame 1. However, the clearance between the radial slide-bearing 2 and the center shaft 3 is so small it acts as a throttle to a provide large flow resistance. Thus, movement of the center shaft 3 toward the closed end of the frame 1 is prevented.
On the other hand, if a vibration or shock is experience which pulls the center shaft 3 in an upward direction, the volume of the space 15 beneath ends 3a and 13a increases. This increase in volume creates an unwanted vacuum when the center shaft 3 is rotated above a certain speed. However, the thrust dynamic pressure developed on the upper surface 13b of the annular thrust plate 13 prevents movement of the center shaft 3 in an upward direction and thus prevents the unwanted vacuum.
Nevertheless, many problems result from providing the above-described pressures which are required to prevent movement of the center shaft 3 during shock or vibration. In particular, the center shaft 3 is subjected to a substantial loss in torque which is undesirable.
Additionally, a number of minute clearances must be defined which require severe manufacturing tolerances that are difficult to obtain. Specifically, a minute clearance must be defined between (i) the lower end 13a of the annular thrust plate 13 and the connected lower end 3a of the center shaft 3, and (ii) the bottom 1b. Moreover, a minute clearance must be defined between the upper end 13b of the annular thrust plate 13 and the lower end 2a of the radial-slide bearing 2 adjacent to the closed end of the frame 1.
Lastly, to develop the thrust dynamic pressures necessary to prevent an unwanted movement of the center shaft 3, grooves must also be formed in opposite sides 13a and 13b of the annular thrust plate 13. Manufacturing these grooves is undesirable given that it results in an increased production cost.